aCal is a standalone calendar application which only supports calendars hosted on a remote CalDAV server.

"Free as in software" with the source code available on gitorious under the GPL v3 license.

The app store version of aCal is sold for a token NZD$2 (or equivalent) to discourage accidental installation by people who don't actually want a CalDAV client, and to help keep the author in beer and pizza.

Troubleshooting

If CalDAV-Sync and CardDAV-Sync don't sync or you cannot set up an account, check the following:

You'll need at least PHP 5.2, for the DateTime class, since CalDAV-Sync seems to trigger the code paths for repeating events even if there are no repeating events in the calendar. (See also Setup_Failure_Codes/PHP_DateTime_class)

Verify that Keep-Alives are enabled in your web server configuration (they're disabled by default in the stock Apache config on RedHat/CentOS/Fedora)

Make sure you didn't change the DAV headers. Go to your DAViCal configuration file and look for a line beginning with

$c->override_dav_header = "..."

If that line exists comment it out or make sure it contains "addressbook" and "calendar-access". CalDAV-Sync and CardDAV-Sync won't sync if a collection is not advertised as calendar or address book by those headers.